Electrical power quality and reliability are critical issues for many electrical power consumers. Computer systems and industrial processing operations can be adversely affected by momentary power disturbances, such as short-term power interruptions and sags, as well as by longer term power outages. Various approaches have been taken to improve quality and reliability. Individual pieces of critical equipment, such a servers, personal computers and other smaller computer systems may be adequately protected by uninterruptible power supplies (UPS) which both condition the power supplied to the consuming equipment and provide short-term backup power in case of a total power outage. Critical power consuming facilities such as hospitals and some industrial plants may have standby power generating equipment which is activated when the main utility power completely fails so as to ensure continued supply of power for as long as necessary to the critical loads. Some large scale electric power consumers that require very high reliability power—for example, semiconductor processing facilities—may be supplied with two independent distribution feeds from the power utility. The feeds are provided from two separate distribution grids, so that a fault occurring on one grid is usually not correlated with a fault on the other grid, making it highly improbable that both utility feeds will see an interruption at the same time. Thus, if a problem is encountered on one utility feed, a fast transfer to the other utility feed allows reliable power to be maintained to the critical loads. Such systems use an open or a closed transition transfer switch to switch power to the facility between the two utility feeds. This approach to providing premium power reliability has been utilized by a variety of large power consumers such as semiconductor and automotive manufacturing plants and large computer data centers.
Although dual utility feed systems do provide relatively high power reliability, power quality problems can still be encountered that are particularly serious for information technology and automated manufacturing processes. One problem relates to the interruption caused by the transfer from one utility feed to the other. A short interruption associated with transferring from one feed to another can result in a long shutdown of a critical industrial process or a computer system, essentially causing an interruption in the utilization of the critical load even though there is no long-term interruption in the supply of power. Relatively slow electro-mechanical transfer switches have the lowest cost and are currently an industry standard, but the use of such switches has the greatest potential for interruption of the power supplied to a critical load and the disruption of the operation of the load. One approach to addressing this problem is by the use of faster transfer switches, including static transfer switches (available from several manufacturers such as Cyberex and Silicon Power), and fast electro-mechanical switches (available, for example, from vendors such as Joslyn). While faster static switches can carry out transfers relatively rapidly to minimize the interruption of the power supplied to the load, the cost of switching systems incorporating such switches is significant.
Another power quality problem which is not well addressed by dual utility feed systems, even with fast transfer switches, results from voltage sags in the power being supplied from the active utility feed to the critical load. The transfer switching system can be set to switch from the sagging utility feed to the other utility feed if the sag becomes significant enough, but with an attendant potential interruption in the power supplied to the load while switching takes place. Thus, it is generally preferable not to trigger a transfer during relatively small voltage sags. However, a voltage sag which is not significant enough to trigger a switch may still affect the performance of the equipment being supplied with the sagging power. Moreover, a significant percentage of voltage sags result from substantial power system faults which cause the sag to propagate over large areas of the utility grid, including the transmission grid. In such cases, even though there may not be a complete outage, the disturbance can extend to both of the utility feeds, and switching from one utility feed to the other does not eliminate the sag problem for the customer.